It's not about decision-making, mostly. It's that there are a smaller number of "good" shots in Marcus' repertoire versus good offensive players. And NBA defenses are likely squeezing Marcus into taking more shots than he wants to (like the way they used to do Rondo).
That is why it is important a lead guard to be a good offensive player even if he doesn't take a lot of shots. In the long run, NBA defenses will force you to your weaknesses. For Marcus, that will mean taking away many of his passing options.